1. Field of the Invention
This invention relates to light interference filters made of alternating layers of tantala and silica and their use on lamps. More particularly, this invention relates to stress relieved interference filters made of alternating layers of tantala and silica having of at least twelve total layers and exhibiting (i) a randomized crack pattern when viewed at 50 times magnification and (ii) diffuse reflection of no greater than about 5% at a wavelength of from 1500-2000 nm, and the use of such filters with electric lamps.
2. Background of the Disclosure
Thin film optical coatings known as interference filters which comprise alternating layers of two or more materials of different indices of refraction are well known to those skilled in the art. Such coatings or films are used to selectively reflect or transmit light radiation from various portions of the electromagnetic radiation spectrum such as ultraviolet, visible and infrared radiation. These films or coatings are used in the lamp industry to coat reflectors and lamp envelopes. One application in which these thin film optical coatings are useful is to improve the illumination efficiency or efficacy of incandescent lamps by reflecting infrared energy emitted by a filament or arc back to the filament or arc while transmitting the visible light portion of the electromagnetic spectrum emitted by the filament. This lowers the amount of electrical energy required to be supplied to the filament to maintain its operating temperature. In other lamp applications where it is desired to transmit infrared radiation, such filters can reflect the shorter wavelength portions of the spectrum, such as ultraviolet and visible light portions emitted by the filament or arc and transmit primarily the infrared portion in order to provide heat radiation with little or no visible light radiation. Such an application of this latter type would include a typical radiant heater for residential or industrial use where visible radiation emitted by the heater is unwanted.
Such interference filters useful for applications where the filter will be exposed to high temperature in excess of 500.degree. C. or so have been made of alternating layers of tantala (tantalum pentoxide Ta.sub.2 O.sub.5) and silica (SiO.sub.2), wherein the silica is the low refractive index material and the tantala is the high refractive index material. Such filters and lamps employing same are disclosed, for example, in U.S. Pat. Nos. 4,588,923; 4,663,557 and 4,689,519. In such lamp applications the interference filters, which are applied on the outside surface of the vitreous lamp envelope containing the filament within, often reach operating temperatures in the range of from about 800.degree.-900.degree. C. These interference filters or coatings have been applied primarily using evaporation or sputtering techniques which, while capable of producing a satisfactory interference filter, has its limitations with respect to not being able to apply a uniform coating to any but a flat surface. In the case of tubing used for making lamps, the tubing must be rotated in the sputtering or vacuum evaporation chamber as the coating is being applied. This technique does not lend itself to applying uniform coatings to curved objects. Moreover, this technique is rather costly.
Consequently, attempts have been made to make such interference filters using a solution deposition technique such as is disclosed in U.S. Pat. No. 4,701,663. In this patent, a film of alternating layers of tantala and silica was not produced. Instead, films were made of alternating layers of titania and silica with the suggestion that these techniques might also apply to tantala-silica films. However, this patent also discloses that severe stresses occur when the titania-silica films are baked at temperatures of from about 500.degree.-600.degree. C., such that the films crack and peel off a silica substrate. The basic thrust of this patent relates to the addition to the silica layers of significant amounts of stress reducing agents, such as phosphorus pentoxide and boron trioxide, which are added in order to reduce the cracking and peeling problem. However, when these additives are added to the silica layers of the filter as taught in this patent, the optical properties are significantly reduced thus resulting in poor filters. Moreover, the addition of such additives to the silica in the significant amounts taught changes the nature of the silica material in the filter to that of a relatively low melting glass.
U.S. Pat. No. 4,734,614 recognizes the problems of severe stress formation associated with the use of interference filters consisting of alternating layers of silica and tantala when employed at high temperatures. Thus, lines 54 et. seq., in column 1 teach that tantala has limited physical and chemical stability and crystallizes to a polycrystalline form after 30 minutes at 800.degree. C. which produces stress cracks visible as craquele. This results in a filter that scatters both visible and infrared radiation, thereby rendering it unsuitable. Niobium pentoxide is suggested as a replacement for the tantala.
Accordingly, a need exists for thin film optical coating interference filters comprising alternating layers of tantala and silica which can be produced by a process which will apply a relatively uniform coating to a complex shape and which does not result in a film containing such stresses that the film cracks and peels off the substrate.